Process Optimization to Prepare High-purity Amorphous Silica from Rice Husks via Citric Acid Leaching Treatment

Rice husks or straws, one of the main agricultural wastes in South-east Asia, have a large possibility to be employed as usefully renewable resources to produce energy and high-purity silica (SiO 2 ), because they consist of about 70% organics such as cellulose or hemi-cellulose, and 20% amorphous SiO 2 . The former is utilized as bio-mass energy; for example, thermal or electric energy, and bio-mass ethanol. High-purity silica can be used as raw materials in industrial application. It would be possible to extract silica elements from rice husks and straws by burning them in air because their main contents are organics. In the previous studies, a strong acid leaching treatment on rice husks before burning was carried out by using H 2 SO 4 , HCl and HNO 3 to prepare high-purity silica materials. This was available to accelerate the hydrolysis of celluloses and hemicelluloses contained in rice husks, and remove the above metallic impurities. A strong acid leaching treatment, however, is significantly hazardous to environment and human life, and causes an increase of the process cost. In this study, an environmentally benign, harmless to human and economically effective process to produce high-purity amorphous SiO 2 materials from rice husks has been established without using strong acids. From a viewpoint of being a harmless influence on the human body, carboxylic acids were used in the leaching treatment on rice husks. TG-DTA measurement and GCMS analysis indicated that the leaching was effective for the hydrolysis of celluloses and hemicelluloses contained in rice husks at 473K∼873K, and produced the same results as using the conventional sulfuric acid. The metallic impurities could be also removed from the husks via a chelate reaction between carboxyl groups (-COOH) and the metal elements. In particular, it was clarified that cadmium (Cd) as completely removed from rice straws by this reaction. Concerning the burning conditions of rice husks after the acid leaching, it was necessary to supply a suitable amount of air to completely combust organics; for example it required an air supplement of 50 ml/min. or more. The burning temperature should be less than 1273K to obtain amorphous structured silica. High-purity amorphous silica materials with 99% or more purity were prepared from rice husks by applying the citric acid leaching treatment and burning process at 1073K in air.